Photographic process



reduce the latter 2,929,709 PHOTOGRAPHIC PROCESS Hendrik Jonker, Roelof Jan Hendrik Aiink, and Tijs Willein van Rijssel, Eindhoven, Netherlands, assignors,'by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Original application July 10, 1951, Serial No. 236,068, now patent No. 2,738,272, dated March 13, 1956. Divided and this application February 15, 1956, Serial No. 565,557

13 Claims. (CI. 96-49) photographic contrasts by means of a sensitized layer containing a mixture of a photosensitive diazo compound and a silver salt which layer after exposure was treated with an alkaline liquid and then with a fixing agent to obtain a silver picture.

The object of this invention is to produce silver pictures having better contrast effects than the above-mentio'ned.

According to the invention a light-sensitive system which is converted upon exposure to light into one or more substances referred to hereinafter as the lightreaction product is reacted with a-silver compound to and form a silver germ image under carefully controlled conditions. The silver germ image is then subsequently intensified by physical development.

In particular the method according to the invention produces improved photographic contrasts by carefully -controlling the conditions leading to the formation of the contrast in a phase intermediate the light-reaction phase and the phase of intensification of the silver germ image, namely a phase of silver germ formation. In this phase a suitable medium is usedto increase the difference in potential between the silver potential and the normallight redox potential 'of'the light reaction product after exposure of the light-sensitive material. The sensitivity of the system is thereby improved to such an extent that at least one-third less light is required to form the silver germ image than that required in the absence of an increase in the diflerence in potential. 'The silver compound necessary to produce said silver germ image may be introduced into a carrier for the light-sensitive material before exposure to light or the silver compound ma be introduced in the carrier for reaction with the "light-reaction product after exposure.

The term light-reaction pro'duct'is hereindefined as the product obtained by the photochemical conversion of light-sensitive substances.

The term silver 'germ image is hereindefined as a'silvernuclei image produced byithe action of the lightreaction product upon a silver compound reducing the latter to metallic silver. The silver germ image corresponds to the latent image in a silver halide system and is weak and invisible to the eye.

The term light redox potential is hereindefined as the potential assumed by a platinum electrode measured with respect to a normal hydrogen electrode in a solution of the light reaction product and of the,,product obtained therefrom upon oxidation thereof by silver ions.

ice

The normal light redox potential is further defined as the potential measured between a platinum electrode and a normal hydrogen electrode in the above solution in which the molecular concentrations of the light-reaction product and the product obtained therefrom upon oxidat-ion bysilver ions are equal to unity. In practice it is suflicient that these concentrations are equal.

The term silver potential is hereindefined as the potential assumed by a silver electrode in a solution containing silver ions measured with respect to a normal hydrogen electrode.

The term physical development within the scope of the invention is to be understood to mean the growth of a weak metallic image to form an image of 'sulhcient photographic density, the image silver produced by reduction at least in part not originating from rare metal ions locally present in crystallised connection.

The term rare metal is to be understood here to mean a metal which is classified above the copper in the electrolytic voltage series of metals. It thus includes inter alia silver, mercury, gold and the platinum metals; in most cases silver is preferred.

It should be noted that the present invention is not applicable to silver halide systems.

In order to obtain better contrasts it has been found that the difference in potential AE between the silver potential and the normal light redox potential must be increased, i.e. the silver potential must be increased and/or the normal light redox potential must be decreased during the formation of the silver germ image. However, not any increase in AB leads to an increase in dispersion of silver in the silver germ image. I It has been found that no appreciable effect can be found in the photographic sensitivity of the system if AB is increased to a value smaller than 0.2 volt during the formation of the silver germ image. Furthermore, it has been found that an increase in AB to a value greater than 0.8 volt no longer results in a greater intensification of the silver germ image.

Exact ascertainment of the reducing power AE prevailing during formation of the silver germ image is not very feasible, since it would require potential measurements in the layer, which are unsuitable in practice. However, as will be seen hereinafter, such measurements are not essential for properly carrying out the methods according tothe invention.

In order to ascertain the AE prior to taking the steps according to the invention the value of the silver potential is determined in a solution in which the molecular concentration of the silver compound is equal to that in the solution by means of which the layer is provided with silver compounds. Then the value of the normal light redox potential is determined in a solution in which the essential conditions are the same as those prevailing in the sensitising solution by means of which the layer is provided with the light-sensitive substance. Consequently, the pH of the measuring solution should be given the same value as the pH of the sensitising solution, whilst substances combining morestrongly with the oxidation product of the light-reaction product than with the light-reaction product itself, if they are present in the sensitising solution, must be added in-the same mov lecular concentration to the measuring solution. Ob- -viously, in performing the'measurements, allowance must bemade for further treatments of the layer (which, as the case may be, are eliected prior to exposure), always provided that said treatments result in an'increase of silver.

.if the pH value remains below 9 and paper serves I I support for the light-sensitive material. Ifregenerated I soluuon containing fern-nitrate and ferro-nitrate m a increase in the difference in potential between the silver potential and the normal light redox potential (AE) results in an increase of the number of germs per unit surface. However, no general relationship between the increase in AB and the increased number of germs can be specified since the relationship is a particular one for each particular system. The relationship is, however, ap plicable for increases in AF. within the limits set forth above.

The normal light redox potential can be reduced by introducing a suitable medium into the system which binds itself to the product resulting from the oxidation of the light reaction product by silver ions to a greater extent than withthe light reaction product itself. This means being a compound selected from the group 'consisting of pyrophosphates,fluorides, citrates, oxalates and tartrates. This means for decreasing the normal light redox potential naturally vary with the light-sensitive substances used. For example, if the light-sensitive substance is a ferric salt, then pyrophosphates, fluorides, and oxalates havethe eflect of decreasing the redox potential of the light reaction product.

The most important method of decreasing the normal light redox potential consists in maintaining the photosensitive layer acidic during exposure and increasingthe pH of the exposed layer by means of a suitable medium, for example an alkaline vapor or liquid or a buffer solution, the pH of which is higher than the pH of the layer. intensification of the silver germ image is accomplished by means of a physical developer containing silver ions, a reducing agent and substances such as acids, alkalies and complexing salts to regulate the rate of development or enhance stability of the solution. Mercury and platinum ions may be substituted for silver ions in the developing solution. Generally acids are used to stabilize .the solution and control the rate of development although alkaline substances and alkali metal sulphites may be used successfully. If citric acid is used forthis stabilizing purpose the contrast becomes a neutral grey color shortly after the beginning of development resulting .in contrasts of satisfactory color having a lower gradation than in contrasts obtained with the use of other developers. The development factor increases as the time of development is longer.

The intensification of the silver germ image by the method according to the invention is so high that the required amount of light required to form silver germ images by the method according to the invention is up to 1000 times less than that used in'methods hitherto known.

If silver compounds spontaneously capable of pro ,viding germs and insoluble silver compounds are deposited at unwanted places they will also be intensified by physical development thereby neutralizing the contrast in the enveloping fog which frequently may have a blackening even higher than 2.

This fogging may be eliminated by treating the silver germ image with a liquid capable of dissolving metallic silver and/or silver compounds, For this purpose treatment with a dilute oxidizing acid, such as nitric acid, or a solution containing ferric cyanide and thiosulphate is particularly suitable. These liquids must only react for a short time and their concentrations must not be unduly high otherwise the silver germ image may also be dissolved.

In cases where the pH of the medium during the formation of the silver germ image is at least temporarily higher than 11, it may be necessary to dissolve metallic However, these baths are preferably limited to those cases in which their use is unavoidable.

If the pH value during the formation of silver germs is not unduly high a solvent may be utilized to dissolve only the silver compounds. Such is generally thecase as the cellulose is used a pH value up to 11 is permissible. By means of this method substantially no silver germs are dissolved during the treatment and only a slight loss in sensitivity occurs. For the removal of silver compounds soluble salts of thiosulphuric acid, thiocyanic acid or sulphuric acid and preferably the alkali metal or ammonium salt'may be used The concentration of the bath and the reaction period should be adjusted to avoid the dissolution of the silver germ image. For example, a 5% aqueous solution of sodium thiosulphate is used to eliminate the fogging of silver compounds by washing for 3 minutes without a resulting loss of sensitivity.

' 'Fogging always occurs if the pH of one of the treatments is greater than 7 to 7.5.

alkaline liquid which can dissolve silver compounds to avoid fogging while simultaneously increasing the pH .of the system. Asolution of thiosulphate which has its pH adjusted to a value between 8 and 12 by the addition of bufiering substances and/or alkalies is particularly suitable for this purpose. A solution of ammonium hydroxide may similarly be used.

The silver compounds may be conveniently added to the sensitising solution used to introduce the light sensitive compound into the layer before exposure.

It is possible to obtain. metallic images having wide limits of gradation, such as values from 0.5 to 10, by a suitable choiceof the silver ion concentration and the pH value during the formation of the silver germ image.

As the silver ion concentration of the bath is increased .the gradation obtained is higher.

In addition the pH value during formation of the silver germ image also appears to have an effect upon the gradation of the conwhich a color contrast in situ with a metal contrast is obtained with the use of physical color developers.

The invention will now be described in connection with the following examples:

Example I Superficially saponified cellulose acetate foil is sensitised by impregnation in a solution containing 0.21 mole of ferric nitrate, 0.23 mole of silver nitrate and 0.15

,mole of citric acid per litre of water, and subsequently dried.

After exposure under a sensitometer with the use of a mercury vapour lamp, the foil is treated for 0.5 minute in a solution containing 0.5 mole of sodium lactate per litre of water (pH=5).

The reducing power (AE) of the light sensitive systern prior to increasing the pH in the layer is 0.75-0.62 volt= 0.13 ,volt (i.e. 0.8 volt). In order to make this clear the following explanation is given. The value 0.75

.is that of the silver potential of the sensitising solution measured in. a solution of 0.23 g./mol. of silver nitrate and 0.15 g./mol. of citric acid per litre water; this value is not varied due to the step concerned. The value 0.62

volt is that of the normal redox potential measured in a solution containingferri-nitrate and ferro-nitrate in a common concentration of 0.21 g./mol. in a litre of 0.15

g./mol. of citric acid solution in water. The use of the step according to the invention, hence by the treatment with a solution containing 0.5 g./mol. of sodium lactate per litre water for 0.5 minute, has the effect of decreasing the normal redox potential from 0.62 to 0.31 volt.

The latter value has been found by measurement on a .is made of paper provided with a seamen e'e'neentr' ti'ea of 0.10 g/jntei. in a time "or "its g/rnel. of sodium aerate solution in water.

After washing, physical development takes place for 5 minutes in a 1% metol-3.5 citric acid-0.40% silver nitrate developer. A copy fof the sensitomete'r of deepblack ca-1e: is obtained, whereas without physical developmenta very vague, yellow copy only is obtained by inuch longer "exposure. The gain in sensitivity as ompared with the'eas'ein which the lactate treatment ist's'initted is at least a factor 30', p

If, instead of utilising the above mention'ed foil, use gelatine layer, a cor respondingiesult' is obtained.

In this example an'd'in both of the other examples use may alternatively be made of physical color develop- 7 cut, for example, a blue green color'may be;produced using a developer containing 0.08% of dimethylam-ine- 4-phenylaminenitra te, 5% of tar taric'acid and 0.4% of silver nitrate to which 2 ccs. of a solution of 10% of naphthol-l in ethanol is added per 25 cos. The silver iina'gemay be'renieved with a li uid containingipotasterricy'anide and thio'sulphate.

Example II Strips of the exposed film of Example I are treated for 0.5 minute in one of the following solutions:

(a) 7.5% of potassium sodium tartrate and 3% of borax in water (pH=9.3),

(b) 5% of soduim pyrophosphate in water (pH=10.1),

(c) 10% of sodium carbonate in water (pH=10.8).

Subsequently, the strips are treated with a 0.3 n solution of ammonia in water for 1 minute. After washing, development takes place in the developer of Example I for 5 minutes. Satisfactory images are obtained, the gain in sensitivity as compared with the cases in which the treatment of the layer for increasing AE during germ formation are omitted, certainly being a factor 100. AE of the light-sensitive system is considerably increased by this treatment, anyway up to a value greater than 0.2 volt. In the base b, for instance AE is certainly increased up to a value of 0.7 to 0.8 volt.

Other strips are bathed for 1 to 2 minutes in the following solutions:

(d) 0.3 n ammonia in water (pH=11.4), (e) 10% of sodium carbonate and 5% of sodium thiosulphate in water (pH=10.8).

After Washing and physical development as indicated above, satisfactory results are likewise obtained, although the gain 1n sensitivity is now only a factor 25 to 60.

Example III A film of superficially saponified cellulose acetate is sensitised by soaking in a solution containing 0.25 mole of uranyl nitrate, 0.23 mole of silver nitrate and 0.08 mole of citric acid per litre water and subsequently dried.

After exposure under a sensitometer with the use of a mercury vapour lamp, a strip is treated in the solution (b) of Example II for 0.5 minute, subsequently bathed in a solution of sodium thiosulphate (10%) in water for 3 minutes and after washing for 7 minutes, physically developed in the developer of Example I for 5 minutes. Another strip is treated in the solution of Example H for 3 minutes. After washing, this strip is also developed in the same developer for minutes. Satisfactory black copies of the sensitometer are obtained. The gain in sensitivity as compared with the method in which physical development only is used is a factor 100 and 25 respectively.

While the invention has thus been described in connection with specific examples and applications thereof, other modifications and substitutions will be obvious to those skilled in the art without departing from the spirit 6 and'seepe ofthe'ihvention as defined in the enters.

What is claimedis:

1. A method of producing photographic contrasts comprising the steps selectively exposing a layer having a 'pHof less than 7.0 and containing a light-sensitive cornpouhd sele'ctedfro'rn the group consisting of water soluble light's'ensitive ferric and uranyl salts to decompose said light-sensitive compound at selected portions of the layerfwhile maintaining the pH of said layer at a'valiie of less than 7.0, treating said exposed layer in combination with 'a water soluble silver salt with an-alkaline 'inedium' capable of increasing the pH of said layer and 'r'r'ioi's'ture to form'a latent'silver contrast, the pH of said alkannemedium being 'sufiicient to increase the difference in potential between the silver potential and the normal light-redox potential of said exposed layertd a value "or "at least 0.2 volt, but less than 0.8 volt, and physicallydeveloping the latent silver contrast by then applying to said layer an aqueous developing solution containing a salt of a'r'net'al higher than copper in the electromotive series and reducible to metal in solution and a water soluble organic reducing agent.

2. A method of producing photographic contrasts comprising the steps selectively exposing a layer having a pH of less than 7.0 and containing a light-sensitive compound selected from the group consisting of water soluble light sensitive ferric and uranyl salts to decompose said lightsensitive compound at selected portions of the layer while maintaining the pH of said layer at a value of less than 7.0, treating said exposed layer in combination with a water soluble silver salt with an alkaline medium capable of increasing the pH of said layer, moisture and a compound which binds itself to a greater extent to the oxidation product obtained by the reaction between the exposed light-sensitive compound and said silver salt than to the exposed light-sensitive compound itself and which is selected from the group consisting of pyrophosphoric acid, hydrofluoric acid, citric acid, oxalic acid and tartaric acid and their water soluble salts to form a latent silver contrast, the pH of said alkaline medium being sufiicient to increase the difference in potential between the silver potential and the normal light redox potential of said exposed layer to a value of at least 0.2 volt, but less than 0.8 volt, and physically developing the latent silver contrast by then applying to said layer an aqueous developmg solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in which reacts with the oxidation product of the exposed light-sensitive compound, together with the pH increasing medium is added to the light-sensitive layer after exposure of the layer. i

6. The method of claim. 2, in which the reducible salt in the developing solution is selected from the group consisting of silver and mercury salts and a color coupler is present in said solution.

7. The method of claim 2, in which any silver or insoluble silver compounds formed in unexposed areas are dissolved before subjecting the system to physical developing.

8. A method of producing photographic contrasts comprising the steps selectively exposing a layer having a pH of less than 7.0 and containing a light-sensitive compound selected from the group consisting of water soluble light sensitive ferric and uranyl salts to decompose said light-sensitive compound at selected portions of the appended layer while maintaining the pH'of said layer at a value of less than 7.0, treating said exposed layer in combination with a water soluble silver salt with an aqueous solution of an alkali metal thiosulfate capable of increasing the pH of said layer, and a compound which binds itself to a greater extent to the oxidation product obtained by the reaction between the exposed light-sensitive compound and said silver salt than to the exposed light-sensitive compound itself and which is selected from the group consisting of pyrophosphoric acid, hydrofluoric acid, citric acid, oxalic acid and tartaric acid and their water soluble salts to form a latent silver contrast, the pH of said aqueous solution of said alkali metal thiosulfate being sufficient to increase the d fierenceinpotential between the silver potential and the normal light redox potential of said exposed layer to a value of at least 0.2 volt, but less than 0.8 volt, and physically developing the latent silver contrast by then applying to said layer an aqueous developing solution containing a salt of a metal higher than copper in the electromotive series and reducible to metal in solution and a water soluble organic reducing agent.

9. The method of claim 8, in which the solution ofthe thiosulphate is adjusted 10. The method of claim 2, in which the pH increasing medium is ammonia. A .3

11. The method of claim 2, in which the pH increasing medium is an aqueous alkaline solution of an alkali-metal thiocyanate. i

'12. The method of claim 2, in which the light-sensitive compound is a water soluble ferric salt. j

13. The method of claim 2, in which the light-sensitive compound is a water soluble uranyl salt.

References Cited in the file of-this patent UNITED STATES PATENTS 1 2,066,918 Poser et al. Jan. s, 1937 2,183,447 Dippel et al. Dec. 12, 19.39

2 2,750,292 7 Dippel et al June 12, 19 56 FOREIGN PATENTS r 232,307 Great Britain Apr. 9, 1925 323,768 Great Britain "Jan. 6, 1930 to a pH value of about 8 to 12., p 

1. A METHOD OF PRODUCING PHOTOGRAPHIC CONTRASTS COMPRISING THE STEPS SELECTIVELY EXPOSING A LAYER HAVING A PH OF LESS THAN 7.0 AND CONTAINING A LIGHT-SENSITIVE COMPOUND SELECTED FROM THE GROUP CONSISTING OF WATER SOLUBLE LIGHT SENSITIVE FERRIC AND URANYL SLATS TO DECOMPOSE SAID LIGHT-SENSITIVE COMPOUND SELECTED PORTIONS OF THE LAYER WHILE MAINTAINING THE PH OF SAID LAYER AT A VALUE OF LESS THAN 7.0 TREATING SAID EXPOSED LAYER IN COMBINATION WITH A WATER SOLUBLE SILVER SALT WITH AN ALKALINE MEDIUM CAPABLE OF INCREASING THE PH OF SAID LAYER AND MOISTURE TO FORM A LATENT SILVER CONTRAST, THE PH OF SAID ALKALINE MEDUIM BEING SUFFICEINT TO INCREASE THE DIFFERENCE IN POTENTIAL BETWEEN THE SILVER POTENTIAL AND THE NORMAL LIGHT REDOX POTENTIAL OF SAID EXPOSED LAYER TO A VALUE OF AT LEAST 0.2 VOLT, BUT LESS THAN 0.8 VOLT, AND PHYSICALLY DEVELOPING THE LATENT SILVER CONTRAST BY THEN APPLYING TO SAID LAYER AN AQUEOUS DEVELOPING SOLUTION CONTAINING A SALT OF METAL HIGHER THAN COPPER IN THE ELECTROMOTIVE SERIES AND REDUCIBLE TO METAL IN SOLUTION AND A WATER SOLUBLE ORGANIC REDUCING AGENT. 